Pluto a planet again?

[skepticalbip]

Perhaps the moon orbiting the Earth makes it a moon or satellite of Earth. Orbiting the Sun in its own right would make it a Planet or planetoid.

All true but what would be required to be identified as a double planet system? Actually the Earth and Moon are both orbiting a common barycenter rather than one of the bodies orbiting the other. Do both bodies need to have the exact same mass to be seen as a double planet system? If it is proposed that the barycenter must be outside the surface of one of the bodies then that should happen eventually as the Moon moves away because of tidal forces slowing the Earth's rotation which would then make the Moon a planet in a double planet system.

 

[DBT]

Perhaps the moon orbiting the Earth makes it a moon or satellite of Earth. Orbiting the Sun in its own right would make it a Planet or planetoid.

All true but what would be required to be identified as a double planet system? Actually the Earth and Moon are both orbiting a common barycenter rather than one of the bodies orbiting the other. Do both bodies need to have the exact same mass to be seen as a double planet system? If it is proposed that the barycenter must be outside the surface of one of the bodies then that should happen eventually as the Moon moves away because of tidal forces slowing the Earth's rotation which would then make the Moon a planet in a double planet system.

I'd say that to qualify as a double planet system, they would have to orbit each other, ie a central point between the two bodies.

 

[lpetrich]

Personally, I don't like the idea of moons being planets. ...

Why so?

 

[steve_bank]

Something like

Solar Objects type
1. Orbiting a star > mass x
2. Orbiting a type 2 object
3. Orbiting the star mass <= x in orbits TBD .(asteroids)
4. Orbiting the star mass <= x in orbits TBD. (comets)


Sub Types
mass
atmosphere
gas
solid
composition

 

[skepticalbip]

Perhaps the moon orbiting the Earth makes it a moon or satellite of Earth. Orbiting the Sun in its own right would make it a Planet or planetoid.

All true but what would be required to be identified as a double planet system? Actually the Earth and Moon are both orbiting a common barycenter rather than one of the bodies orbiting the other. Do both bodies need to have the exact same mass to be seen as a double planet system? If it is proposed that the barycenter must be outside the surface of one of the bodies then that should happen eventually as the Moon moves away because of tidal forces slowing the Earth's rotation which would then make the Moon a planet in a double planet system.

I'd say that to qualify as a double planet system, they would have to orbit each other, ie a central point between the two bodies.

The Earth and Moon do both orbit around their barycenter. The barycenter is about 2,900 miles from Earth's center and about 240,000 miles from the Moon's center.

 

[DBT]

Perhaps the moon orbiting the Earth makes it a moon or satellite of Earth. Orbiting the Sun in its own right would make it a Planet or planetoid.

All true but what would be required to be identified as a double planet system? Actually the Earth and Moon are both orbiting a common barycenter rather than one of the bodies orbiting the other. Do both bodies need to have the exact same mass to be seen as a double planet system? If it is proposed that the barycenter must be outside the surface of one of the bodies then that should happen eventually as the Moon moves away because of tidal forces slowing the Earth's rotation which would then make the Moon a planet in a double planet system.

I'd say that to qualify as a double planet system, they would have to orbit each other, ie a central point between the two bodies.

The Earth and Moon do both orbit around their barycenter. The barycenter is about 2,900 miles from Earth's center and about 240,000 miles from the Moon's center.

Nowhere near enough to qualify. The centre should be a point in space between the two planets.

 

[steve_bank]

And the Sun moves around the center of mass of the solar system. The Earth and moon are going round the galaxy, the galxy is moving through space.

The practical reference point is the center of mass of the solar system. It stays constant as the planets move around the Sun.

Astronomical coordinate systems - Wikipedia

en.wikipedia.org

Ecliptic system​

Main article: Ecliptic coordinate system
The fundamental plane is the plane of the Earth's orbit, called the ecliptic plane. There are two principal variants of the ecliptic coordinate system: geocentric ecliptic coordinates centered on the Earth and heliocentric ecliptic coordinates centered on the center of mass of the Solar System.

The geocentric ecliptic system was the principal coordinate system for ancient astronomy and is still useful for computing the apparent motions of the Sun, Moon, and planets.[3]

The heliocentric ecliptic system describes the planets' orbital movement around the Sun, and centers on the barycenter of the Solar System (i.e. very close to the center of the Sun). The system is primarily used for computing the positions of planets and other Solar System bodies, as well as defining their orbital elements.

A planet travels in the ecliptic.

 

[skepticalbip]

And the Sun moves around the center of mass of the solar system. The Earth and moon are going round the galaxy, the galxy is moving through space.

The practical reference point is the center of mass of the solar system. It stays constant as the planets move around the Sun.

Astronomical coordinate systems - Wikipedia

en.wikipedia.org

Ecliptic system​

Main article: Ecliptic coordinate system
The fundamental plane is the plane of the Earth's orbit, called the ecliptic plane. There are two principal variants of the ecliptic coordinate system: geocentric ecliptic coordinates centered on the Earth and heliocentric ecliptic coordinates centered on the center of mass of the Solar System.

The geocentric ecliptic system was the principal coordinate system for ancient astronomy and is still useful for computing the apparent motions of the Sun, Moon, and planets.[3]

The heliocentric ecliptic system describes the planets' orbital movement around the Sun, and centers on the barycenter of the Solar System (i.e. very close to the center of the Sun). The system is primarily used for computing the positions of planets and other Solar System bodies, as well as defining their orbital elements.

A planet travels in the ecliptic.

If that's the criteria then it is another reason to kick Pluto out of the family of planets. Pluto's orbit is inclined 17 degrees out of the ecliptic. But then Ceres does orbit in the ecliptic.

 

[Loren Pechtel]

They proposed

1. Be low enough in mass that at no time (past or present) can it generate energy in its interior due to any self-sustaining nuclear fusion chain reaction (else it would be a brown dwarf or a star). And also,
2. Be large enough that its shape becomes determined primarily by gravity rather than mechanical strength or other factors (e.g. surface tension, rotation rate) in less than a Hubble time, so that the body would on this timescale or shorter reach a state of hydrostatic equilibrium in its interior.

They divide planets into

• überplanets orbit stars and are dynamically dominant enough to clear neighboring planetesimals in a Hubble time
• unterplanets, which cannot clear their neighborhood, for example are in unstable orbits, or are in resonance with or orbit a more massive body.

 Clearing the neighbourhood - several criteria. A simple one is relative mass of satellite -- (its mass) / (its primary's mass)

(Question, though: Is it possible to have planets orbit each other in the long term?)

The Earth-moon system has persisted for quite a while. The Moon is significantly larger than Pluto and only slightly smaller than Mercury. I guess for clarity, would the Moon be considered to be a planet if it were in an independent orbit about the Sun. If so then then the Earth-Moon system could be called a dual planet system.

There's an awful lot more Earth than Moon, I wouldn't call it a dual planet system.

 

[Loren Pechtel]

Perhaps the moon orbiting the Earth makes it a moon or satellite of Earth. Orbiting the Sun in its own right would make it a Planet or planetoid.

All true but what would be required to be identified as a double planet system? Actually the Earth and Moon are both orbiting a common barycenter rather than one of the bodies orbiting the other. Do both bodies need to have the exact same mass to be seen as a double planet system? If it is proposed that the barycenter must be outside the surface of one of the bodies then that should happen eventually as the Moon moves away because of tidal forces slowing the Earth's rotation which would then make the Moon a planet in a double planet system.

The barycenter will move away from Earth before the Moon gets too far out and gets ripped off at some point?

 

[steve_bank]

And the Sun moves around the center of mass of the solar system. The Earth and moon are going round the galaxy, the galxy is moving through space.

The practical reference point is the center of mass of the solar system. It stays constant as the planets move around the Sun.

Astronomical coordinate systems - Wikipedia

en.wikipedia.org

Ecliptic system​

Main article: Ecliptic coordinate system
The fundamental plane is the plane of the Earth's orbit, called the ecliptic plane. There are two principal variants of the ecliptic coordinate system: geocentric ecliptic coordinates centered on the Earth and heliocentric ecliptic coordinates centered on the center of mass of the Solar System.

The geocentric ecliptic system was the principal coordinate system for ancient astronomy and is still useful for computing the apparent motions of the Sun, Moon, and planets.[3]

The heliocentric ecliptic system describes the planets' orbital movement around the Sun, and centers on the barycenter of the Solar System (i.e. very close to the center of the Sun). The system is primarily used for computing the positions of planets and other Solar System bodies, as well as defining their orbital elements.

A planet travels in the ecliptic.

If that's the criteria then it is another reason to kick Pluto out of the family of planets. Pluto's orbit is inclined 17 degrees out of the ecliptic. But then Ceres does orbit in the ecliptic.

My point is soft philosophical definitions versus quantifiable parameters and definitions..

 

[bilby]

They proposed

1. Be low enough in mass that at no time (past or present) can it generate energy in its interior due to any self-sustaining nuclear fusion chain reaction (else it would be a brown dwarf or a star). And also,
2. Be large enough that its shape becomes determined primarily by gravity rather than mechanical strength or other factors (e.g. surface tension, rotation rate) in less than a Hubble time, so that the body would on this timescale or shorter reach a state of hydrostatic equilibrium in its interior.

They divide planets into

• überplanets orbit stars and are dynamically dominant enough to clear neighboring planetesimals in a Hubble time
• unterplanets, which cannot clear their neighborhood, for example are in unstable orbits, or are in resonance with or orbit a more massive body.

 Clearing the neighbourhood - several criteria. A simple one is relative mass of satellite -- (its mass) / (its primary's mass)

(Question, though: Is it possible to have planets orbit each other in the long term?)

The Earth-moon system has persisted for quite a while. The Moon is significantly larger than Pluto and only slightly smaller than Mercury. I guess for clarity, would the Moon be considered to be a planet if it were in an independent orbit about the Sun. If so then then the Earth-Moon system could be called a dual planet system.

There's an awful lot more Earth than Moon, I wouldn't call it a dual planet system.

Well I am glad you have developed such a clear and precise criterion for that decision.

A dual planet system is one where there's possibly more of one body than the other, but not an awful lot more.

No room for disagreement there.

 

[Jokodo]

Perhaps the moon orbiting the Earth makes it a moon or satellite of Earth. Orbiting the Sun in its own right would make it a Planet or planetoid.

All true but what would be required to be identified as a double planet system? Actually the Earth and Moon are both orbiting a common barycenter rather than one of the bodies orbiting the other. Do both bodies need to have the exact same mass to be seen as a double planet system? If it is proposed that the barycenter must be outside the surface of one of the bodies then that should happen eventually as the Moon moves away because of tidal forces slowing the Earth's rotation which would then make the Moon a planet in a double planet system.

Strictly speaking, the Earth and the ISS also orbit a common baryocentre.